The present invention relates to the production of ceramic materials or glass by high frequency induction melting in a furnace, on whose walls form an insulating crust or autocrucible.
In general terms, it is known that ceramic oxides, which are good electrical insulants at ambient temperature, have a resistivity .rho. which decreases with increasing temperature (approximately 0.1 to 10 Ohm.cm at around their liquefaction temperature).
It is therefore possible to maintain these materials in the molten state by induction heating at a high frequency, e.g. approximately 100 to 500 KHz, provided that the materials are previously raised to an adequate temperature for bringing about their liquefaction and that the furnace is given the necessary minimum dimensions for obtaining a correct electric induction heating throughout the molten mass.
In the known processes of this type, the materials to be melted are generally placed in a good heat-conducting (generally copper) pot or crucible, whose walls are cooled by a circulation of water and externally surrounded by a helical coil through which passes the high frequency inducing current bringing about the heating of the central mass contained in the pot by electromagnetic induction. Due to the powerful cooling of the cylindrical copper walls forming the pot, a crust or skin forms internally against said wall and brings about a thermal and electrical insulation of the hot liquid part located within the crust and where all the induced energy is given off. In the known equipment of this type, it is necessary to work with conventional high frequency generators and also in an intermittent manner, i.e. for each operation the pot must be filled with powder containing the different components of the material to be produced, followed by induction heating, emptying its liquid phase and cleaning before the following operation.
Moreover, the fact that the inducing helical coil is separate from the copper crucible leads to a significant high frequency power loss (approximately 50%) and the discontinuous nature of the production leads to a by no means negligible energy consumption due to the successive preheatings of the material obtained either by introducing good electricity-conducting products into the mass, or by direct heating with external means, such as e.g. combustion gases.
Consideration has therefore been given to the improvement of the energy utilization of such induction furnaces by forming the wall of the crucible by the actual primary inductor and the secondary of the thus formed electrical transformer is constituted by the molten material mass, within which induced currents develop.
This applies with respect to the electric furnace described in French Pat. No. 1,430,192, which essentially comprises a cylindrical metal wall, slotted along a generatrix and sealed by an insulating joint 2 (FIG. 2) made from a sufficiently refractory material for the metal wall to form a single turn coil connected on either side of joint 2 to two poles of a high frequency power supply.
However, a furnace of this type suffers from two serious disadvantages. Firstly, the slot made in the cylinder constituting the furnace wall produces a high magnetic field gradient, which is prejudicial to the homogeneity of the inductive heating. Secondly, the single turn coil formed in this way can only be supplied by the high frequency generator across an air-core transformer, which leads to a significant energy loss and to a correlative reduction in the efficiency of the installation.